An alkaline storage battery such as a nickel-metal hydride battery or nickel-cadmium battery has conventionally employed a sintered nickel electrode or a non-sintered nickel electrode as its positive electrode.
The non-sintered nickel electrode is produced by directly applying an active material paste mainly containing nickel hydroxide to a conductive porous body such as foamed nickel. Although it can be easily produced, a disadvantage exists that it is poor in charge-discharge characteristics at high current.
On the other hand, the sintered nickel electrode employs a porous sintered nickel substrate obtained by sintering and is produced by chemically impregnating the porous sintered nickel substrate with a salt of the active material. The sintered nickel substrate presents higher conductivity. In addition, the electrode has excellent charge-discharge characteristics at high current because of good adhesion of the active material to the porous sintered nickel substrate. On this account, an alkaline storage battery employing the sintered nickel electrode has been favorably used in an electric power tool requiring high current discharge.
Unfortunately, however, the sintered nickel electrode has a lower applying ratio of the active material than the non-sintered nickel electrode and therefor, must be improved in the utilization of the active material. In addition, in an alkaline storage battery employing the sintered nickel electrode, the above-mentioned sintered nickel substrate becomes weak due to the repeated charging and discharging of the battery. This results in low charge-discharge cycle characteristics of the battery.
On this account, there has been conventionally proposed a sintered nickel electrode wherein a layer composed of cobalt hydroxide is formed on a surface of an active material applied to a porous sintered nickel substrate, after which the layer is heat-treated in the presence of oxygen and an alkaline solution so that the cobalt hydroxide improve the conductivity of the active material thereby improving the utilization thereof, as disclosed in JP, 1-200555, A. Also, there has been proposed a sintered nickel electrode wherein a layer composed of cobalt hydroxide is formed on a surface of a porous sintered nickel substrate, after which the layer is heat-treated in the presence of oxygen and an alkaline solution, and an active material mainly containing nickel hydroxide is then applied to the above-mentioned sintered nickel substrate, to inhibit the corrosion of the sintered nickel substrate during the application of the active material so that charge-discharge cycle characteristics of the battery is improve, as disclosed in JP, 63-216268, A.
Unfortunately, however, even in a case where the sintered nickel electrode produced in the manner disclosed in the above-mentioned JP, 1-200555, A is used as a positive electrode of an alkaline storage battery, the alkaline storage battery still suffers the occurrence of self discharge due to the oxygen evolution in the sintered nickel electrode when the battery in a charged state is stored at a high temperature of approximately 50° C. for a long time. Thus, the alkaline storage battery is reduced in capacity.
Also, even in a case where the sintered nickel electrode produced in the manner as disclosed in the above-mentioned JP, 63-216268, A (JP, 5-50099, B) is used as a positive electrode of an alkaline storage battery, the oxygen evolution occurs in the alkaline storage battery charged at a high temperature of approximately 50° C. before the positive electrode is charged to full. As a result, the battery is decreased in charge efficiency.
Further, there have been proposed a sintered nickel electrode wherein a positive-electrode active material contains yttrium hydroxide to improve the utilization thereof under high temperature conditions as disclosed in JP, 48-50233, A, and a sintered nickel electrode wherein a compound such as of yttrium, indium, antimony or the like is added to an active material mainly containing nickel hydroxide to improve the utilization thereof under high temperature conditions as disclosed in JP, 5-28992, A.
However, in each of the sintered nickel electrodes disclosed in these official gazettes, the compound such as of yttrium is simply added to the active material. Therefore, the active material and the sintered nickel substrate are not sufficiently covered with the compound such as of yttrium. This detrimentally allows electrolyte solutions contact with the active material and/or the sintered nickel substrate. Accordingly, oxygen evolution still occurs in the sintered nickel electrode under high temperature conditions, and sufficient increase in the utilization of the active materials thus can not be achieved.
An object of the present invention is to provide, in an alkaline storage battery employing as its positive electrode a sintered nickel electrode comprising a porous sintered nickel substrate having an active material mainly containing nickel hydroxide applied thereto, the alkaline storage battery with excellent storage characteristics under high temperature conditions by suppressing self-discharge due to the oxygen gas evolution in the above-mentioned nickel electrode even when the battery in a charged state is stored at high temperature for a long time.
Another object of the invention is to provide, in an alkaline storage battery employing as its positive electrode a sintered nickel electrode comprising a porous sintered nickel substrate having an active material mainly containing nickel hydroxide applied thereto, the alkaline storage battery with a sufficient battery capacity under high temperature conditions by suppressing oxygen evolution before the above-mentioned nickel electrode is charged to full when the battery is charged under high temperature conditions.